Process for roasting solids

ABSTRACT

Solids such as dolomite or magnesia are continuously roasted in a vertical furnace provided with an axial tube to cause the heating flame to pass through the center of the solids, the heating being effected by the combustion of dry fuel oil vapors preheated to 300* to 650* C.

United States Patent Inventors Andre Alphonse Frltsch Garches; Maxl-lermant Hicguet, Boulogne- Billancourt, both of France Appl. No.845,260 Filed July 28, 1969 Patented Dec. 7, 1971 Assignee SocieteDEtudes Et de Recherches Sclentifiques Et Minleres Malakotl, Hauls dcSel'ne, France Priorities July 30, 1968 France 161 153; July 10, 1969,France, No. 6923514 PROCESS FOR ROASTING SOLIDS 9 Claims, 12 DrawingFigs.

[ 263/52 [51] F27b 1/10 [50] Field 01 Search 263/29, 30.

[56] References Cited UNITED STATES PATENTS 3,17 l ,637 3/1965 Gribbin263/29 3,427,367 2/1969 Kiehl 263/52 Primary Examiner-John .I. CambyAttorney-Stevens. Davis, Miller & Mosher ABSTRACT: Solids such asdolomite or magnesia are continuously roasted in a vertical furnaceprovided with an axial tube to cause the heating flame to pass throughthe center of the solids, the heating being effected by the combustionof dry fuel oil vapors preheated to 300 to 650 C.

PATENTED nEc Yuan 3525497 sum 2 OF 8 PATENTEU DEC Hen 3.625497 SHEET 3[IF 8 L jg 2a; Z8 Z0 PATENTED DEB 7197i SHEET 5 [1F 8 PATENTED DEC 7197iSHEET 8 OF 8 PATENTEDDEC Han 3,625,497

SHEET 8 [1F 8 PROCESS FOR ROASTING SOLIDS This invention relates to theroasting of solids such as dolomite or magnesia, and to furnaces for usein such roasting.

In any vertical furnace the phenomenon which in blast fur naces iscalled the wall effect arises. Voids exist between the wall and thelumps forming the outer part of the charge while at the center of thefurnace the lumps enmesh and prevent the rise of gas. For this reasonthe flame in vertical furnaces always rises on the outside of thecharge.

One of the great difficulties which is encountered in heating a furnacewith a liquid or gaseous fuel is to achieve deep and uniform penetrationof the heating fluid into the interior of the charge and to produce auniform mixture of the combustion air and of the fuel used.

In order to combat the wall effect, attempts have been made to usechimneys which are extended downwards by a tube or dipping chimney whichenters the charge but because of variations in draught considerablevariations in roasting arise therefrom. Hanging and sticking occurinside the furnace and there is also a high production of material whichhas not been roasted in spite of all the means employed, especially theadditional introduction of air in order to split up the draught in thechimney to a greater or lesser extent.

The present invention provides a process and furnace which makes itpossible to sinter dolomite and magnesia or to roast lime with a verylow consumption of fuel and with the uniformity of roasting which isindispensable for obtaining sintered dolomite, sintered magnesia, limeor any other product which can be roasted in vertical furnaces. Inparticular the invention provides a process for producing sintereddolomite, sintered magnesia, or lime or high purity while avoiding theircombination with fuel ashes.

In its broadest aspect the invention provides a process for continuouslyroasting a solid in a vertical furnace which corn prises introducing dryfuel oil vapors in admixture, at a temperature of 300 to 650 C. with airin an amount less than 40 percent of the air required for the totalcombustion of the said fuel oil vapors, into the furnace to be burntthere and thereby roast the said solid.

In one particular embodiment of the invention, the material to beroasted, for exampledolomite, magnesia, calcium carbonate or chamotte isintroduced and guided in a vertical tubular chamber in such a way thatit initially descends therein in the shape of a ring of which the centeris free for the rise of the gas, and then in the form of a solid column,and so that gas rising through the mass encounters more resistance onrising at the periphery than at the center.

The furnace of the invention for carrying out the new process comprisesvents combined with a device for introducing dry fuel oil vapors andair. For carrying out the new process the furnace is preferablyprovided, at the top and centrally, with a vertical tube which is openat the top and bottom without being joined to the chimney. This tubemakes it possible to introduce the solid to be roasted as a ring whilebelow the bottom of the tube the solid assumes the shape of a fullcolumn. Thus any gas rising in this column encounters more resistance incontinuing its path in the ring formed between the tube and the top ofthe wall of the furnace than in continuing its path at the center. Atthe same time it is possible to adjust the height of the annular portionof the charge in such a way that an equilibrium is established betweenthe gas travelling along the wall of the furnace and the gas passingthrough the center.

it is furthermore necessary to combat another phenomenon which is thedescent of the flame inside the furnace. in effect, the wall in avertical furnace restricts the descent of the solids so that thesetravel more easily at the center, and this tends to accentuate the walleffect. For this purpose, according to the invention, a device forwithdrawal of material from a furnace is provided which comprises a flathearth located under the furnace and avoiding the crumbling which takesplace if voids are produced in the bottom of the charge. Such voids ineffect allow the charge to descend more or less rapidly and if there ismaterial which has been less roasted on one side, subsidences occurwhich completely destroy the equilibrium of the flame zone andconsequently of the roasting.

The device for withdrawing material from the furnace can moreparticularly comprise a hearth of which the peripheral part is flatwhile the central part is domed for better direction and restriction ofthe descent of materials at the center. In particular, the device can bea revolving hearth with a central cone. The hearth may be provided onits circumference with teeth intended to engage with the teeth of anannular set of teeth which is fixed or, if appropriate, revolves aroundthe hearth.

The new process for roasting at a high temperature in a verticalfurnace, more particularly dolomite, magnesia, calcium carbonate orchamotte, has been conceived to avoid the disadvantages encountered ifsuch a furnace is heated with producer gas or even with atomized fueloil.

One serious disadvantage is the formation of an extremely hard cokewhich welds together the lumps of the solid which is roasted, and as aresult prevents the mass from descending, and blocks the nozzles orvents for the introduction of fuel, in some cases in a few minutes. Thesame is true if hot fuel oil is used, for example at a temperature ofthe order to C. in an atomized fonn, even in extremely fine droplets ofthe order of a micron.

Now the applicants have found that this disadvantage can be avoided if amixture, at a temperature of 300 to 400 C., or even, in appropriatecases, as high as 650 C. if a fuel oil very rich in carbon, such asBunker C or tar" is used, of dry vapors of fuel oil and of air in anamount less than 40 percent of the amount necessary for the completecombustion of the fuel oil, is introduced into the furnace. The balanceof the air for supporting combustion or secondary air is introducedseparately, the combustible mixture being preferably introduced into thelower third of the furnace and the secondary air at the bottom.

in order to produce the mixture of dry fuel vapors and primary air, thefuel oil can first be heated to a temperature of the order of 300 to 350C. under a pressure of about 10 bars and then expanded to atmosphericpressure or to a slightly higher pressure, for example about 0.5 bars,resulting in the production of vapors and in a drop in temperature ofthe order of l00C., and the vapors finally reheated while air is addedin an amount which is insufficient to render the mixture producedinflammable. For example, I to 5 percent of the amount of air requiredfor the combustion may be added, and a supplement up to a total of lessthan 40 percent can be added subsequently before admission to thefurnace. The mixing may be carried out in a tube into which thehydrocarbon vapors pass, and which contains a tube with a parallel axis,which introduces hot primary air in the direction of flow of the vaporsthrough a multiplicity of holes, allowing the addition to be carried outgradually.

In order to heat the liquid fuel oil as well as the mixture of fuel oilvapors and air and, if appropriate, the primary air, it is possible,according to the invention, to operate with indirect exchange with aheating fluid" which can be heated to a boiler also running on fuel oil.By heating fluid" is meant a liquid of high specific heat which boils ata high temperature and has a large interval between its freezing pointand its boiling point, such as is found in commerce for indirect heatingto temperatures exceeding 200 C. and able to range up to about 400 C. Asexamples, biphenyl derivatives and natural oils used as such orhydrogenated, such as perhydrosqualene, may be mentioned.

The amount of primary air added to the fuel vapors may be regulated as afunction of the desired speed of ignition.

In parallel to the introduction of the mixture of fuel vapor and airunder a slight pressure, the establishment of pressure reduction in thefurnace is provided, for example by suction at the outlet.

The description which follows with reference to the accompanyingdrawings is given by way of example.

FIG. 1 is a transverse section of a portion of a known vertical furnace.

FIG. 2 is a partial vertical section of such a furnace on a smallerscale.

FIG. 3 which is similar to FIG. 2, schematically shows the addition of atube in accordance with the invention.

FIG. 4 shows the upper part of the furnace provided with a particularcharging device in addition to this tube, under the same conditions asFIG. 3 but on a larger scale.

FIG. 5 schematically shows the bottom of the furnace and the hearth forwithdrawing material from the furnace, in a vertical section.

FIG. 6 is a schematic view, again in a vertical section, of a portion ofa furnace constructed in accordance with the invention and equipped forbeing heated with a liquid fuel.

FIG. 7 is a part section along VII-VII of FIG. 6.

FIG. 8 represents a similar furnace with a double fuel feed, on asmaller scale.

FIG. 9 is a schematic representation of the feed of liquid fuel andcombustion-supporting material to the furnace.

FIG. 10 is an elevation view, in axial section, of a portion of theimproved vertical furnace.

FIG. 11 is a simplified plan view showing a section along XIXI of FIG.10.

FIG. 12 which is similar to FIG. 11, shows a variant of the shape of thecombustion antechamber.

FIGS. 1 and 2 relate to known furnaces. The refractory lining is marked1 and the charge of material to be roasted is marked 2; between thelumps of this material and the lining there exist voids 3, which join oralmost join, and which define a quasi-continuous ring of lesserresistance to the rise of the gas. It is from this that the wall effectresults, with the zone of maximum temperature or flame zone 4 (FIG. 2)having a tendency to rise along the lining l as is indicated in 4a. Itwill also be seen that below the flame zone 4 the roasted materialdescends nonuniformly, and faster at the center than along thecircumference (line 4b defines the bottom of the flame zone).

According to one of the characteristics of the invention, a tube 5 (FIG.3) which is open at the top and at the bottom is suspended coaxially inthe upper part of the furnace so as to assist the gas in issuing throughthe center, and hence to counterbalance the wall effect. The maximumtemperature zone which in the case of dolomite comprises thedecarbonation zone 60 and the roasting or sintering zone 6b, instead ofrising at the periphery and being depressed at the center, ispractically at the same level in these various areas; this makes itpossible to obtain completely uniform roasting with considerable economyin fuel.

The use of the dip tube 5 makes it possible considerably to reduce theheight of the flame zone which, as a result, utilizes the heat providedby the fuel much better while making more regular roasting possible.Furthermore, comparison of the flame zone 6a, 6b of FIG. 3 with the zone4 of FIG. 2 proves that the height of the furnace can, thanks to the diptube, be considerably reduced, which makes it possible correspondinglyto reduce the wall effect which has to be combated in a furnace in viewof the fact that the higher a furnace is, the greater will be the walleffect and the more difiicult it will be to combat it. Conversely, ifone is dealing with a shorter furnace, it is possible to pass the gasmore uniformly through the entire mass and to achieve better roasting.

As also emerges from FIG. 3, the particular feature residing in theaddition of the tube 5 is combined with the feature of placing thelargest lumps 7 of the material to be roasted, for example lumps ofdolomite of size about 50 to 60 mm., at the center, and placing thesmallest lumps 8, for example lumps of dolomite of size about 30 to 50mm., at the periphery. The tube 5 can, when charging, be used to providethe separation between small and large lumps and, where appropriate, tomaintain a spacing between the lower surfaces of the correspondingstacks, as has been assumed in the drawing. The gas which rises in thefurnace thus finds a passage of lesser resistance at the center, acrossthe large lumps, than it does across the small lumps, and then a passageof practically zero resistance in the tube.

FIG. 4 schematically shows a device for introducing material into thefurnace which comprises, above the tube 5, a chute 9 so mounted as torevolve about its axis coaxially with the tube 5, which receives thelarge lumps of material to be roasted in order to direct them into thetube, in combination with a side chute 10 of which the outlet section10a opens above the annular space between the tube 5 and the inner wallof the furnace; the chute 10 serves to introduce the smaller lumps.

The furnace can be provided with level indicators which via controllersoperate the introduction of the lumps which are to be roasted. Bycoupling the effect of the suction caused by the tube 5 and thedifference in level between the small lumps located outside this tubeand the large lumps located at the center, these level controllers makeit possible to provide a compensation of the wall effect and to pass thegas uniformly over the entire surface of the furnace. In order to causemore gas to pass to the center, it suffices to provide a higher level ofthe small lumps on the outside of the tube, and vice versa. It is alsopossible to change the ratio of the average sizes of the large lumps andthe small lumps. Any other method of charging which allows the smalllumps to be placed on the outside and the large lumps at the center canbe used.

The method for charging which has just been described is preferablycombined with a device for removing material from the furnace whichprevents the charge from descending more rapidly at the center than atthe sides and which, as shown schematically in FIG. 5, comprises ahearth of which the peripheral part 20 is flat and the central part 21is bulging, for example in the form of a cone. In order to control thedescent of the charge better and to restrict it in the middle, the wallI of the furnace may at its bottom possess a metal collar 22 of largerinternal diameter; as a result of this the charge spreads and travelsmore slowly and there is no danger of the collar projecting when thebricks of the wall 1 of the furnace have been spent. Due to the factthat the charge arrives in this secton of the furnace in a partly cooledstate, the collar can be made of cast iron or even of sheet iron.

The hearthmay be fixed; but it preferably revolves as shown in thedrawing. To do this, it is connected to a central shaft 25 which iscoupled to a drive shaft 26 by a gear wheel 24 and a worm 28. The cone21 may be grooved on its surface and the peripheral portion 20 of thehearth may be provided at 20a with teeth intended to act on the materialin conjunction with a fixed set of teeth 27. Sufficient play, forexample two and a half times the size of the large lumps to be removedfrom the furnace, is provided between the hearth and the set of teeth 27so that the material which has sintered and not stuck can descendfreely. If, in spite of the absence of fuel ashes, sticking occurscaused, e.g., by excessive heat or the presence of soil from the quarry,the fixed and movable teeth perform the function of a grinder byseparating the lumps which have simply stuck together and whichgenerally become unstuck fairly easily. In the case of lumps which havebeen abnormally welded together and are too hard, such lumps would beretained on the edge of the set of teeth 27 and could be broken by thepersonnel in charge of the furnace.

The collar 22 can itself be provided with vertical teeth, grooves orroughenings which resist the charge, driven by the revolving hearth,assuming a rotating movement at its bottom which could interfere withthe removal of material from the furnace and could wear the lining.

The furnace operated with fuel oil vapor which is shown in FIGS. 6 and 7comprises, in its upper part, a tube 30 arranged as has been describedabove, and at 31 a refractory lining is shown which, if one is forexample dealing with a furnace for roasting dolomite, surrounds thedecarbonation zone and, below it, the sintering zone of the dolomite. Itis at the bottom of the latter that the mixture of fuel oil vapors andprimary air is introduced.

To do this, the portion of the furnace 32 which is located below thesintering zone and which preferably has a slightly larger diameterpossesses one or several rows of vents. A horizontal row of four vents33a has here been shown. A tube 34a for introducing the mixture opensinto each vent. The tube 34a widens out as it enters (FIG. 7). at thelower part of the furnace FIG. 6 again shows a collar 35 and a revolvinghearth 36 similar to those which have been described above.

The secondary air enters through the bottom of the charge resting on thehearth and rises across the mass while becoming heated, until it reachesthe mixture issuing from the vents. It is frequently advantageous toprovide, close to the vents which serve to admit the mixture of fuel andprimary air, additional vents which are allotted to the introduction ofsupplementary hot secondary air. In the present case it is for examplepossible to provide, below the row of vents 33a, a row of similar vents33b which receive hot air through the tubes 34b.

It is advantageous for the distance of the vents 330 from the bottom ofthe tube 30 to represent about half the height of the charge in thefurnace counted from the hearth 36, as this allows the combustion gas topenetrate to the very center of the charge, thus suppressing the walleffect.

in the case of FIG. 8, a similar arrangement is again found and the samereference numbers have been used. However the furnace shows additionalfittings for obtaining particularly high temperatures. In the case inquestion, the internal wall of the furnace has orifices 37 slightlyabove the middle point of its height, these being for the admission ofcombustion gas which provides additional heat, for example fordecarbonating the dolomite, and as a result of this the material to beroasted reaches the sintering zone in a hotter condition and thetemperature can thus there rise further under the influence of thecombustion of the fuel oil vapors.

More particularly, a combustion pocket or chamber 38 can be provided asa source of hot gas opposite each orifice 37, wherein volatilehydrocarbons which are thus relatively rich in hydrogen and which areintroduced through a jet 39 are completely burned. More particularly, itis possible to use the distillation head fractions of heavy fuel oil,with the liquid hydrocarbons collected at the sump being intended forthe production of the vapor, as has been stated above. The combustiongas issuing from the pockets 38 and entering the furnace can be attemperatures of the order of l,200 to l,800 C.

An advantage of this procedure is that while benefiting from the heatwhich the hydrocarbons of relatively high hydrogen content compared totheir carbon content are capable of providing, one avoids burning themin the hottest regions of the furnace, that is to say one avoidsproducing there a large amount of water vapor which is harmful to thelinings of a basic material joined together by pitch and which lowersthe temperature.

It is of value to provide a fan 6 above the furnace, which produces asuction at that point and which draws up the gaseous combustion productsand, where relevant, the gas resulting from the decomposition of thecarbonates contained in the solids which are treated.

The mixture of fuel oil vapors and primary air can advantageously beproduced in the manner which will now be described in relation to FIG.9.

The liquid fuel, introduced into a tank 40, is passed by means of a pump43 and a hose 4i provided with a filter 42 into a heater 44 where it isprovided with heat by a heating fluid flowing through a coiled portion450 of a tube 45. The pressure in the heater may be of the order of barsand the temperature of the order of 350 C. A tube 46 is provided with anexpansion valve 47 and ending in a second heater 48 starts from theheater 44. In the second heater 48 the fuel valve produced by theexpansion receives heat from the heating fluid through another coiledportion 45b of the tube 45.

Upstream from the heater 48 there enters a tube 53 coming from an aircompressor 51 and passing through a heater 52 which receives heat from athird coiled portion 45c of the tube 45. The heater 48 is followed by amixer 49 heated by a fourth coiled portion 45d of the tube 45. The endportion 50a of a tube 50 is arranged in the mixer 49 parallel to itsaxis; this portion 50a, running in the direction of flow of the vapors,is perforated with holes to allow hot air to be gradually introducedinto the hydrocarbon vapors. At the outlet of the mixer 49 the vapors towhich hot primary air has thus been added are introduced into aheat-insulated collector 55. From the collector 55 run the tubes 34amentioned above, which open into the vents of the furnace 56. Anoptional tube 54 is provided with a stopcock 57 which allows additionalhot primary air to be introduced, through its branches 54a, 54b, 54c and54d, at the vents of the furnace.

As shown in FIGS. 6 and 7, the mixture of fuel oil vapors and primaryair is introduced into the furnace through the vents in a zone ofslightly greater diameter than the zone above it (the sintering zone inthe case of dolomite), so that at the periphery of the material whichhas just been roasted a subsidence slope forms, leaving above it anannular chamber which allows the fuel oil vapors accompanied by primaryair to mix better with the secondary air coming from the bottom of thefurnace, and allows the mixture thus formed to distribute itself betterin the furnace. Additionally, in FIGS. 6 and 7, the vents which serve toadmit the mixture of fuel oil vapors and primary air and, where present,the supplementary vents which allow additional hot secondary air to beintroduced, have been shown as positioned in a similar row under the rowof first vents. Both have been shown as opening into parts which havebeen recessed or cut into the wall of the furnace.

One difficulty is regulating the furnace is to run it in such a way thatthe roasting temperature is kept essentially constant and so that therise of the flame in the charge is controlled.

Since the amount of primary premixed air is 40 percent less than thatrequired for the total combustion of the fuel oil, it is an amount of atleast 60 percent of the air, and hence a markedly larger amount, whichenters at the bottom of the furnace and cools the charge while itselfbecoming heated. If the flame rises, the charge which the secondary airencounters has been able to cool further, and as a result the secondaryair arrives in a less hot condition at the level of the vents which thecombustible mixture issues. Air which is hotter than 300 C. allows thetemperature of the flame to be increased by about l00 C. If the amountof secondary air represents percent of the total amount of combustionair, and if the temperature of this secondary air is raised by 600 C.the temperature of the flame can be increased by about 600X().75/3 thatis to say about l50 C., which is very appreciable.

Preferably the furnace of the invention possesses, below the roastingzone and at the top of a zone of greater diameter than the latter, ventsintended for the introduction of a hot mixture of dry fuel vapors andprimary air, where appropriate during the combustion, as has beendescribed above, and also in its inner wall, under each vent, anessentially vertical cut starting from the bottom of this wall andopening into the said vent, combined with a device which allows hot air,in an amount which can be varied as desired and, where appropriate, at atemperature which can also be varied as desired, to be introduced intothe material which has been roasted, vertically below each cut. I

The furnace is preferably subjected to a sufficient suction at the topfor a draught of air to be produced at the base of extraction of thechange which has passed through the furnace, and the hot air can itselfbe introduced at atmospheric pressure or, preferably, at a higherpressure. As a result of this particular feature of the invention, thehot air reaches the vent outlets through preferred passages and makes itpossible to obtain more or less than hot flames at these outlets andhence to regulate the roasting.

' In order that the cuts should have the minimum chance of being blockedby lumps of roasting material, it is advantageous to make them not morethan 20 cm. wide; a width of the order of l0 cm. is generally suitablebut the width can be less, for example of the order of 5 cm. or less, ifthe roasted material is friable.

It is recommended that the vents are made in the form of loopholes witha width not exceeding that of the cuts below them and preferably lessthan this, so that no ridge should be created for the materialsdescending in the furnace to rest against, and so that there should beno risk of the vents becoming blocked. The chambers opening into thefurnace through the vents can run so as to widen out towards theperiphery of this furnace.

This improvement is described in more detail with reference to FIGS.10-12.

The furnace represented in FIG. 10 and intended for sintering dolomiteis similar to that of FIG. 6. It comprises a cylindrical portionconsisting of an inner refractory lining of basic bricks 101 made ofmagnesia or dolomite, of an intermediate layer 102 which is also basic,and an outer heat insulating layer 103, made of acid bricks, the wholebeing encased in a metal sheet 104. This cylindrical portion rests onthe floor of the factory by means of posts 105.

The upper part of this cylindrical portion defines, as has beenindicated above, a decarbonation zone (which is not shown and which atthe top comprises a dip tube such as the tube 30 of FIGS. 6 and 7) and,below, a sintering zone of which the bottom is seen at 106. The lowerpart, marked 107, is of greater diameter, for example by 10 cm., thanthe upper part, At the top, the lower part 107 has a row of vents 108,in the form of narrow slits or loopholes which are for example only 3 tocm. in width, while their height may be of the order of 50 to 100 cm.The vents may be more or less numerous; there may for example be eightof them (as is shown) or more, with up to 16 where appropriate. Theymay, where appropriate, be divided between two superposed rows in placeof only one. In view of their narrowness the risk of roast materialentering is minimal. The vents represent the mouth of antechambers 109which, as shown in FIG. 11, can be in the approximate shape of truncatedpyramids, that is to say they can be laterally delimited by the planes109a and radiating from the axis of the furnace. Thus for example anantechamber 109 opening into the furnace through a vent of 5 cm. widthhas a width of 24.3 cm. at its external end if the furnace has anexternal diameter of 3.80 cm. and an internal diameter of l.80 m. By wayof a variant (FIG. 12), the antechamber may be delimited by radiatingwalls 110a and 110b in the confines of the first row of bricks and thenwiden out further, at least in the initial portion 110:, 110d of thesewalls, so as to create a turbulence zone for any fluid flowing throughthe antechamber from the outside towards the vent.

Into each antechamber 109 there opens from the outer side, a tube 111which serves to introduce a mixture of dry fuel oil vapor and primaryair, one of these fluids being introduced into the central tubular pipe111a and the other into the peripheral pipe lllb. Thus the vapor canstart to burn in the antechamber and the combustion continues with theaid of secondary air introduced at the antechamber outlet, or partlybefore the outlet and partly at the outlet. The amount of primary airemployed can in practise represent 5 to 50 percent of the amount of airrequired for the total combustion of the fuel oil.

As shown in FIGS. 10 and 11, a slit or cut 112 is produced in theportion 107 of the furnace under each of the vents 108. The width of thecut is equal to or preferably, as shown, greater than that of the ventso that there is no shelf at the bottom of the vent which would providea point of rest for the roasted material descending in the furnace andwould thus favor the blocking of the vent. On the contrary, any lump ofmaterial which entered the vent would find a void below it as a resultof the cut, and would drop. The depth of the cuts can in practice be ofthe order of 10 cm. but can be different, generally greater.

Under the furnace proper, a discharge collar 113 can advantageously beprovided, and this can for example be made of cast iron or optionally ofsheet metal and has a diameter equal to or greater than the diameter ofthe portion 107 calculated to the bottom of the cuts 112. As has beendescribed above, a hearth with a flat peripheral part 1140 and a domedcentral part 114b, for example in the shape of a cone, is located underthe collar 113; this hearth can be fixed or, preferably, revolving, asdescribed above, and its drive shaft is represented at 115.

A tube 116, opening into the collar 113, for introducing hot air intothe charge of material which has just been removed from the furnace ispreferably provided vertically below each cut 112. Because of the factthat the cuts are essentially unblocked and as a result of the walleffect, it is in these cuts that the suction applied at the top of thefurnace primarily makes itself felt, so that the hot air introducedthrough each tube 116 tends to rise selectively towards the cut above itand hence to reach the corresponding vent 108 as fast as possible and bythe shortest route, in order to feed the flame there.

The tubes 116 may be fed by means of a ring-shaped collector 117 which,for convenience, can for example be located around the furnace, at theheight of the nozzles 111 or below, around the collar 113 as shown, orin some other position.

The hot air can come from any appropriate installation. Such aninstallation can in particular be an installation for the production ofdry fuel oil vapors mixed with the primary air for the heatingrequirements of the furnace, as has been described in relation to FIG.9. According to this example, air compressed by a compressor 50 isintroduced into a heater 52 which receives heat indirectly from aheating fluid; air can then be drawn from the outlet of this heaterthrough a control device (shown schematically in the form of a valve 118in the tube 119 feeding the collector I17) and supplied in amounts whichcan be varied as desired, where appropriate with addition of cold air ina controlled amount, to the tubes 115. If the air coming from the heaterS2 is not at a sufficiently high temperature to be introduced into thebottom of the charge of roasted material, it can be further heatedbefore being used.

In the installation for the production of dry fuel oil vapors mixed withprimary air in accordance with the example illustrated in FIG. 9, thevapors to which primary air has been added, on issuing from the mixer49, are introduced into an intermediate heat-insulated collector 55 fromwhere the mixture is passed towards the vents of the furnace through thetubes 34a.

The applicants have found that in practice the collector can be omitted,and that the tube-which ends there and comes from the mixer 49 candirectly and in parallel feed the tubes 340 which for example run to thejets 111; these tubes 34a may themselves subdivide if they are fewer innumber than the jets.

The device thus described can be repeated above the sintering zone (inthe case of dolimite), this zone being put in place slightly to reducethe temperature of the combustion gases having traversed the sinteringzone and for this reason partially dissociated, to permit them torecombine and thus produce a second, automatic heating zone differentfrom the principal zone.

We claim:

1. Process for continuously roasting a solid in a vertical furnace whichcomprises heating liquid fuel oil to a temperature of the order of 300to 350 C. under a pressure of the order of 10 bars,

lowering the pressure to atmospheric pressure or to a slightly higherpressure, for example about 0.5 bar, to produce fuel oil vapors,

heating the fuel oil vapors so obtained and adding hot air thereto in anamount less than 40 percent of the air required for the total combustionof the said fuel oil vapors, before, during or after the heating of saidfuel oil vapors, and

introducing the mixture of fuel oil vapors and air thus produced, at atemperature of 300 to 650 G, into the furnace to be burnt there andthereby roast the said solid.

2. Process according to claim 1 in which, after heating the fuel vaporsand adding part of the hot primary air, additional hot primary air isgradually added to the mixture produced.

3. Process according to claim 1 in which combustion gas is introduced ata high temperature at a level above that at which the fuel oil vaporsand primary air are introduced.

4. Process according to claim 3 in which the combustion gas is theproduct obtained by completely burning the distillation head fractionsof fuel oil outside the roasting chamber of the furnace.

5. Process according to claim 3 in which the combustion gas isintroduced below or at the bottom of the decarbonation zone if the solidbeing roasted is a carbonate.

6. Process according to claim 1 in which the solid to be roasted in thefurnace is introduced and guided in such a way that it first descends asa ring and then as a solid column, so that gas rising through the massencounters more resistance on rising at the periphery than at thecenter, appropriate temperatures for roasting the solid are establishedin the lower part of the column, and the roasted solid is withdrawn atthe bottom of the column.

7. Process according to claim 6 in which the ring is formed from smallerlumps of the material to be roasted than those of which the central coreof the column is made up.

8. Process for continuously roasting a solid in a vertical furnace,which comprises feeding said solid as a mixture of lumps to the top ofsaid furnace, guiding said solid in such a way that it first descends asa ring and then as a solid column, maintaining a suction at the top ofsaid ring to cause air to pass as an updraft from the bottom of saidcolumn through said column and said ring, passing fuel oil through aheating zone at a temperature of the range 300350 C. and under apressure of the order of 10 bars, passing the hot fuel oil through azone where a lower pressure prevails so as to produce fuel oil vapors,passing the fuel oil effluent from the last-named zone through areheating zone, feeding said preheated fuel oil effluent together withair in an amount less than 40 percent of the air required for the totalcombustion of said fuel oil effluent, laterally into the lower part ofsaid column, and withdrawing roasted solid from the bottom of saidcolumn.

9. The process of claim 8, wherein said preheated fuel oil effluent,together with air is fed into said column through a plurality ofcircumferentially spaced apart, vertical narrow passages.

2. Process according to claim 1 in which, after heating the fuel vaporsand adding part of the hot primary air, additional hot primary air isgradually added to the mixture produced.
 3. Process according to claim 1in which combustion gas is introduced at a high temperature at a levelabove that at which the fuel oil vapors and primary air are introduced.4. Process according to claim 3 in which the combustion gas is theproduct obtained by completely burning the distillation head fractionsof fuel oil outside the roasting chamber of the furnace.
 5. Processaccording to claim 3 in which the combustion gas is introduced below orat the bottom of the decarbonation zone if the solid being roasted is acarbonate.
 6. Process according to claim 1 in which the solid to beroasted in the furnace is introduced and guided in sucH a way that itfirst descends as a ring and then as a solid column, so that gas risingthrough the mass encounters more resistance on rising at the peripherythan at the center, appropriate temperatures for roasting the solid areestablished in the lower part of the column, and the roasted solid iswithdrawn at the bottom of the column.
 7. Process according to claim 6in which the ring is formed from smaller lumps of the material to beroasted than those of which the central core of the column is made up.8. Process for continuously roasting a solid in a vertical furnace,which comprises feeding said solid as a mixture of lumps to the top ofsaid furnace, guiding said solid in such a way that it first descends asa ring and then as a solid column, maintaining a suction at the top ofsaid ring to cause air to pass as an updraft from the bottom of saidcolumn through said column and said ring, passing fuel oil through aheating zone at a temperature of the range 300*-350* C. and under apressure of the order of 10 bars, passing the hot fuel oil through azone where a lower pressure prevails so as to produce fuel oil vapors,passing the fuel oil effluent from the last-named zone through areheating zone, feeding said preheated fuel oil effluent together withair in an amount less than 40 percent of the air required for the totalcombustion of said fuel oil effluent, laterally into the lower part ofsaid column, and withdrawing roasted solid from the bottom of saidcolumn.
 9. The process of claim 8, wherein said preheated fuel oileffluent, together with air is fed into said column through a pluralityof circumferentially spaced apart, vertical narrow passages.